1. Field of the Invention
The present invention relates to an internal combustion engine provided with an intake bypass control device. More particularly, the present invention relates to an internal combustion engine provided with an exhaust gas turbocharger and an intake bypass control device adapted to drive a compressor by means of exhaust gas for intake air charging.
2. Description of the Related Art
Internal combustion engines such as gasoline engines and diesel engines that are provided with an exhaust gas turbocharger are known. The exhaust gas turbocharger revolves a turbine by utilizing the pressure of exhaust gas from the internal combustion engine and also drives a compressor by the torque of the turbine for intake air charging the internal combustion engine. Internal combustion engines provided with an exhaust gas turbocharger and also with a bypass circuit that allows the outlet passage of the compressor and the inlet passage of the turbine to connect with each other are also known (see, for example, Japanese Patent Laid-Open Publication No. 2001-165000, p 9–10, FIG. 1.)
Internal combustion engines provided with the bypass circuit are designed to efficiently driving an exhaust gas recirculation system of the engine, or EGR system. An EGR system reduces the oxygen concentration in the intake air by partly circulating the exhaust gas to the intake air of the internal combustion engine and lowers the combustion temperature of the internal combustion engine in order to suppress the generation of nitrogen oxides (NOx) in exhaust gas. Internal combustion engines provided with the EGR system are so designed that the emission rate of NOx is observed and exhaust gas recirculation is made to take place when the emission rate becomes higher than a predetermined level. If the intake pressure of the internal combustion engine is higher than the exhaust pressure and exhaust gas hardly flows to the intake side, the bypass circuit is opened to allow the intake air to partly flow into the exhaust passage and reduce the intake pressure in order to facilitate the recirculation of exhaust gas. With this control arrangement, it is possible to efficiently conduct the EGR.
Meanwhile, in the case of the engine of a construction machine such as a hydraulic power shovel, there are occasions where a high output is required in a low-speed operation (a high low-speed torque). When an exhaust gas turbocharger is combined with an engine that is used for a construction machine and a highly efficient operation point that is found at the smaller flow rate side relative to the choke flow rate of the compressor is used as rated point of the engine, the operation point of the compressor can be found near the surging range where the compressor operates unstably particularly when the compressor is driven to operate in a low-speed and high-load region.
Particularly, in the case of an engine that is provided with the EGR system as described above, the turbocharger is required to charge the engine with a high pressure ratio in order to secure a volume of intake air that matches the required output level because exhaust gas is partly fed back into the intake air to reduce the volume of intake air. However, a high pressure ratio turbocharger type compressor tends to reduce the region between the choke flow rate that is in the operable flow rate range and the surging flow rate when it is in operation. Thus, it is difficult to find an exhaust gas turbocharger that matches the engine.
The use of a wide range type compressor may be conceivable to avoid a combination of an engine and an exhaust gas turbocharger that make the operation point of the compressor approach the surging range in a low-speed and high-load region. However, wide range type compressors normally show a low pressure ratio and are not adapted to a high rotational speed. Therefore, a wide range type compressor is not suited for high pressure ratio supercharging.